Don't limit ExternalName::TestName length #4764
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In order to keep the `ExternalName` enum small, the `TestcaseName` struct was limited to 17 bytes: a 1 byte length and a 16 byte buffer. Due to alignment, that made `ExternalName` 20 bytes. That fixed-size buffer means that the names of functions in Cranelift filetests are truncated to fit, which limits our ability to give tests meaningful names. And I think meaningful names are important in tests. This patch replaces the inline `TestcaseName` buffer with a heap-allocated slice. We don't care about performance for test names, so an indirection out to the heap is fine in that case. But we do care somewhat about the size of `ExternalName` when it's used during compiles. On 64-bit systems, `Box<[u8]>` is 16 bytes, so `TestcaseName` gets one byte smaller. Unfortunately, its alignment is 8 bytes, so `ExternalName` grows from 20 to 24 bytes. According to `valgrind --tool=dhat`, this change has very little effect on compiler performance. Building wasmtime with `--no-default-features --release`, and compiling the pulldown-cmark benchmark from Sightglass, I measured these differences between `main` and this patch: - total number of allocations didn't change (`ExternalName::TestCase` is not used in normal compiles) - 592 more bytes allocated over the process lifetime, out of 171.5MiB - 320 more bytes allocated at peak heap size, out of 12MiB - 0.24% more instructions executed - 16,987 more bytes written - 12,120 _fewer_ bytes read
cfallin
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Aug 24, 2022
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This leads to much cleaner-looking test files -- thanks a bunch!
total number of allocations didn't change (ExternalName::TestCase is not used in normal compiles)
592 more bytes allocated over the process lifetime, out of 171.5MiB
320 more bytes allocated at peak heap size, out of 12MiB
Yeah, I think that's fine; 593 extra bytes on 171.5MiB would have been just far too much, but 592, I think I can live with :-)
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Following up on discussion in #4667, here's a way to avoid making sweeping changes while still using reasonable names in filetests. @afonso360, how does this look to you? @cfallin, do the numbers here sound okay to you?
In order to keep the
ExternalNameenum small, theTestcaseNamestruct was limited to 17 bytes: a 1 byte length and a 16 byte buffer. Due to alignment, that madeExternalName20 bytes.That fixed-size buffer means that the names of functions in Cranelift filetests are truncated to fit, which limits our ability to give tests meaningful names. And I think meaningful names are important in tests.
This patch replaces the inline
TestcaseNamebuffer with a heap-allocated slice. We don't care about performance for test names, so an indirection out to the heap is fine in that case. But we do care somewhat about the size ofExternalNamewhen it's used during compiles.On 64-bit systems,
Box<[u8]>is 16 bytes, soTestcaseNamegets one byte smaller. Unfortunately, its alignment is 8 bytes, soExternalNamegrows from 20 to 24 bytes.According to
valgrind --tool=dhat, this change has very little effect on compiler performance. Building wasmtime with--no-default-features --release, and compiling the pulldown-cmark benchmark from Sightglass, I measured these differences betweenmainand this patch:ExternalName::TestCaseis not used in normal compiles)